Sound amplification system



2 Sheets-Sheet 1- M. R. SCHROEDER SOUND AMPLIFICATION SYSTEM /NVENTOP M.R. SCHROEDE/P A7' 7 ORNE Y May l1, 1965 Filed March 7, 1962 UnitedVStates Patent O '3,183,304 SOUND AMPLIFECATION SYSTEM Manfred R.Schroeder, Gillette, N J., assigmor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York 'Filed Mar. 7,1962, Ser. No. 178,190 Claims. (Cl. 179-1) This invention relates tosound amplication systems, and in particular to sound amplificationsystems whose loudspeakers and microphones are located in the same soundfield.

For a sound amplification system having a microphone and a loudspeakerlocated in the same sound field, for example, a public address system inan auditorium or a distant talking telephone system in an office, thewellknown phenomenon of acoustic feedback between loudspeaker andmicrophone ordinarily prevents full use of the amplification of whichthe system is capable. Acoustic feedback occurs whenever an amplifiedsound emitted by a loudspeaker reaches the microphone, therebyestablishing a feedback loop comprising the sound amplification systemand the room. However, instability of the sound amplification system dueto acoustic feedback occurs only when the loudness of the feedback soundreaching the microphone cumulatively increases on each trip around thefeedback loop. In the absence of preventive measures, the cumulativeincrease in loudness results first in an audible singing noise, andfinally in instability and a breakdown of the system, which is evidencedby a howling noise that obliterates the sound that it is desired toamplify.

Of the various types of feedback sound that cau-se instability in soundsystems, one of the most universal is reverberant sound, which is causedby refiection from the room back :to the microphone of sound emitted bya loudspeaker. Reverberant sound presents a universal problem because ofthe inherent nature of the so-called frequency response curves ofordinary rooms, the frequency response curve of an ordinary room beingcharacterized by an irregular succession of several thousand closelyspaced peaks and valleys within the frequency range of audible sound.For a sound amplification system operating at a sufiiciently high gainlevel in such a room, there are generally one or more components of theemitted sound which coincide wtih peaks in the response curve, therebysustaining the loudness of these components on reflection back to themicrophone. On each trip around the feedback loop, the loudness of thesecomponents cumulatively increases, ultimately resulting in acousticfeedback instability.

One well-known method of preventing acoustic feedback instability causedby reverberant sound is to shift the frequency of each component passingthrough the sound system by a small, nearly imperceptible amount, on theorder of two to five cycles per second. This frequency shift moves eachreverberan-t component to a different point on the frequency responsecurve of the room on each trip around the feedback loop, thus making ithighly probable that after several trips around the loop eachreverberant component will have coincided with or fallen near a valleyin the frequency response curve which will attenuate rather than sustainthe loudness of each reverberant component. However, this method doesnot permit full use to be made of the amplification capabilities of asound system because in the short but significant interval of time whichis required for the frequency shift to become effective, the loudness ofsome reverberant components is cumulatively increased by an amountsufficient to cause a subjectively unpleasant singing noise. To preventthis singing noise, the gain of the system must be maintained at a levelsubstantially below that which is theoretically attainable withfrequency shifting.

The present invention provides another solution to the problem ofacoustic feedback instability, which permits a sound amplificationsystem to be operated in an ordinary room or auditorium at a gain verynear the theoretically attainable maXimum without risk of instability.In this invention, there is inserted between the microphone and thepower amplifier of a sound system a frequency shifter followed by aso-called suppressor- The 'frequency shifter changes the frequency ofeach incoming sound component by a relatively small amount, and thefrequency shifted components are applied to the suppressor. In thesuppressor, the incoming components are separated from each other on thebasis of frequency, and only those components whose amplitudes exceed apredetermined threshold are allowed to pass through the suppressor tothe power amplifier. By appropriately adjusting the threshold, onlycomponents having relatively large amplitudes pass to the amplifier andall other conlponents are suppressed. Since the componentsrof directsound, that is, sound received directly from a desired source such as aspeaker, are characterized by amplitudes which are large relative to theamplitudes of reverberant sound, the suppressor operates to eliminatethe vreverberant components, thereby preventing acoustic feedbackinstability.

The frequency shifter and the suppressor of the present invention areeffective in preventing acoustic feedback instability in each of thethree situations thatare possible when reverberant sound is detected bythe microphone of a soun-d amplificatori system: first, the reverberantsound coincides in time with a later portion of the same direct soundfrom which the reverberant sound is derived; second, the reverberantsound coincides in time with a direct sound other than the direct soundfrom which the reverberant sound is derived; and third, the reverberantsound is not coincident in time with any other sound.

In the first and most dicult situation, the frequency shifter enablesthe suppressor to separate reverberant and direct sound components,despite the fact that the reverberant and the direct componentsoriginate from the same sound. The reverberant components differ infrequency from the direct components because of the change in frequencyintroduced by the frequency shifter during'the passage of the earlierpart of the direct sound through the system, the reverberant sound beingderived from the earlier part of the direct sound. In addition, becausethe reverberant components are generally smaller in amplitude than thecorresponding direct components, owing to attenuation by the air andrefiection from the walls of the room en route to the microphone, thesuppressor, with its appropriately adjusted threshold, eliminates thesmaller reverberant components and passes the larger direct components.Some direct components of small amplitude are also eliminated by thesuppressor, but most of the information content ofV sound is Vconveyedby the largest components, hence intelligibility is not impaired throughthe loss of these small amplitude direct components.

Although the suppressor is able to distinguish between components thatlie relatively close to one another on the frequency scale, occasionallyin the first situation a reverberant component and its correspondingdirect component are not separated on the first trip of thel reverberantcomponent around the feedback loop, in which case the combined amplitudeof the reverberant component and the corresponding direct component isgenerally large enough to exceed the suppressor threshold, thus enablingthe reverberant component to pass through the suppressor together withthe direct component to the power amplifier. However, on each successivetrip around the feedback loop, the frequency shifter preceding thesuppressor Patented May l1, 1965 3 increases the frequency differencebetween a reverberant component and its corresponding direct componentuntil after two or three trips, the suppressor is able to separate thetwo components and thereby eliminate the smaller reverberant component.

Further, each successive change in frequency introduced by the frequencyshifter moves a reverberant component to a different point on the roomfrequency response curve, thereby increasing the probability that areverberant component will coincide with a valley in the curve and beeliminated. The frequency shifter therefore performs two functions: toseparate a smaller reverberant component from its corresponding largerdirect component in order to enable the suppressor to eliminate thereverberant component; and to move a reverberant component to adifferent point on the frequency response curve of the room on each triparound the feedback loop in order to eliminate it by making it coincidewith or fall near a valley in the response curve.

In the second and third situations, the present invention is even moreeffective in preventing acoustic feedback instability. When reverberantsound coincides in time with a direct sound other than the direct soundfrom which the reverberant sound is derived, it is only in randominstances that a reverberant component lies sufiiciently close on thefrequency scale to a direct component for the suppressor to be unable toseparate all reverberant components from all direct components;therefore, most reverberant components will be eliminated in thesuppressor on their first trip around the feedback loop in the secondsituation. In the third situation, when the reverberant sound does notcoincide with any direct sound, there are, of course, no direct soundcomponents which would enable reverberant components to pass through thesuppressor in the fashion described above; hence all reverberantcomponents are eliminated by the suppressor on their first trip aroundthe feedback loop.

The invention will be fully understood from the following detaileddescription of an illustrative embodiment thereof taken in connectionwith the appended drawings, in which:

FIG. 1 is a schematic block diagram showing a sound amplification systemembodying the principles of this invention;

FIGS. 2A, 2B, 2C, and 2D are amplitude spectrum diagrams of assistancein explaining the apparatus of FIG. 1; and

FIG. 3 is a section of the frequency response curve of a typical room orauditorium.

Referring first to FIG. 1, there is shown a sound amplification systemembodying the principles of this invention, in which both transducer 1and reproducer 6 of the systern are located in a typical room orauditorium characterized by a highly irregular frequency response curveof the type shown in FIG. 3. Any sound within the room that is detectedby transducer 1, which may be a microphone of conventional design, isconverted into an electrical wave, and this wave is delivered tofrequency shifter 3.

Frequency shifter 3 is of well-known construction, and it operates tochange the frequency of all the components of the wave by a uniform,predetermined amount, Af cycles per second, where an appropriate valuefor/Af may be on the order of ten. The output terminal of frequencyshifter 3 is connected to the input terminal of suppressor 4, and withinsuppressor 4, the incoming Wave is applied in parallel to delay element47 and to 11 identical subpaths, Where the 1'th subpath comprisesbandpass filter 411, rectifier 42, low-pass filter 431', gate 441', andbandpass filter 451', all connected in series. Bandpass filters 4111through 4111 are provided with relatively narrow, contiguous pass bandsof uniform width which span the frequency range of the incoming wave. Asuitable Width may lie in the range between thirty and three hundredcycles per second; for example, with a width of one hundred cycles persecond, an incoming wave whose frequency range extends from one hundredto ten thousand cycles per second requires subpaths, as indicated inFIG. l. Filters 4111 through 4111 separate the incoming wave into itsindividual frequency components, and rectifiers 42a through 4211followed by low-pass filters 4311 through 4311 develop, in wellknownfashion, a set of control signals representatives of the amplitudes ofthe frequency components passed by the preceding bandpass filters.

Since the amplitudes of the frequency components of a sound constitutethe amplitude spectrum of the sound, the control signals appearing atthe output terminals of low-pass filters 4311 through 4311 represent theamplitude spectrum of a sound. The amplitude spectrum of a typicalvoiced speech sound before passing through frequency shifter 3 isillustrated graphically in FIG. 2A, where the heights of the regularlyspaced vertical lines indicate the amplitudes of the various frequencycomponents that occur at harmonics of the so-called fundamentalfrequency, fg, of the sound. The same spectrum after passage throughfrequency shifter 3 is shown in FIG. 2B, where the broken vertical linesindicate the original components and the solid lines indicate thefrequency shifted components.

Gates 4411 through 4411 following low-pass filters 43a through 4311 areconventional linear gates of identical construction, and each gate isprovided with a control terminal, an input terminal, and an outputterminal. Each of these gates operates in well-known fashion so thatwhen a sufficiently large control signal is impressed upon its controlterminal, any signal simultaneously applied to its input terminal isimmediately passed in substantially unaltered `form to its outputterminal.

In the present invention, the output terminals of lowpass filters 43athrough 4311 are connected to the control terminals of gates 4411through 4411, respectively, and the output terminal of delay element 47is connected in parallel to all of the input terminals of gates 44athrough 4411. By suitably adjusting the bias of each of the gates 44athrough 4411 to respond only to those control signals whose magnitudesexceed a sufficiently high, predetermined threshold, the incoming wavefrom delay element 47 will appear only at the output terminals of thosegates that receive control signals representing frequency componentswhich occur at peaks or formants of the amplitude spectrum of theincoming wave. Similarly, since bandpass filters 4511 through 4511,which are connected to the output terminals of gates 44a through 4411,respectively, have pass bands that are identical with the pass bands ofcorresponding bandpass filters 41a through 4111, the signals appearingat the output terminals of filters 45a through 4511 will be thefrequency components that occur at the formants of the amplitudespectrum of the incoming wave from delay element 47.

It is Well known that the frequency components which Occur at spectralformants of a sound convey most of the information content of the sound,hence the elimination of all but the largest frequency components bysuppressor 4 does not impair the intelligibility of the sound. Thecomponents passed by filters 45a through 4511 are combined in aconventional adder 46 to form a reconstructed wave which is amplified inpower amplifier 5 and reproduced as audible sound in the room byreproducer 6, which may be a conventional loudspeaker.

If desired, the reconstructed wave from adder 6 may be passed toamplifier 5 through delay element 2 instead of directly by settingswitch S to the appropriate position. In this way, a delay intervaladditional to that created by passage through suppressor 4 may beintroduced, this additional delay serving to separate still further intime reverberant sound from direct sound. This delay helps tosynchronize the reception of a sound by distant listeners in a largeauditorium with its utterance by a speaker, and in the case of extendedor sustained sounds, it also helps to reduce the likelihood thatreverberant sound will coincide at the microphone with the same'directsound from which it originated, thereby improving the effectiveness ofthis invention in preventing acoustic feedback instability. Asuitabledelay interval may be on the order of ten milliseconds. v

The relationship of asuitable gate threshold to the amplitudes of thefrequency components of a typical sound is illustrated by the horizontalbroken line in FIG. 2B, where it is observed that the amplitudes of onlyfour components exceed the threshold. The spectrum of the wavereconstructed by adder 46 from these four components is shown in FIG.2C. It is apparent from FIGS. 2B and 2C that the number of componentsappearing in the spectrum of the reconstructed wave may be increased ordecreased by appropriately adjusting the gate threshold, the number ofcomponents shown in FIG. 2C being merely illustrative of the principlesof this invention.

The sound reproduced by loudspeaker 6 from the reconstructed wave andemitted in an ordinary room is typically reflected back to microphone lover various feedback paths, thereby establishing a feedback loep Withinthe sound amplification system and the room. Some 0f the possiblefeedback paths in the room shown in FIG. l are indicated by broken linesconnecting loudspeaker 6 with microphone l..

However, not all of the components of the sound emitted by loudspeaker 6appear in the reverberant sound reaching microphone 1 because thosecomponents of the emitted sound that coincide in frequency withsubstantial valleys in the frequency response curve of the room are tooattenuated by reflection to be detected by microphone 1. In addition,even those components that coincide with peaks in the frequency responsecurve and are therefore sufliciently strong to be detected by microphone1 are generally smaller in amplitude than when they were emitted byloudspeaker 6, owing to losses on transmission through the air and onreflection from the Walls of the room. Thus, the components ofreverberant sound arriving at microphone ll coincide in frequency butnot in amplitude with components of sound emitted by loudspeaker 6, andsome components of emitted sound do not have any counterparts inreverberant sound. In addition, it is to be noted that because of thechange in frequency introduced by frequency shifter 3, the reverberantcomponents do not coincide in frequency with components of the originaldirect sound but are shifted on the frequency scale by an amount equalto Af cycles per second.

In discussing the operation of the present invention in preventingacoustic feedback instability due to reverberant sound, there are threepossible situations of interest when reverberant sound reachesmicrophone l: in the first situation, the original direct sound fromwhich the reverberant sound is derived is still being produced by thesource, as in the case of a sustained vowel or a musical sound; in thesecond situation, the original direct sound from which the reverberantsound is derived has stopped and has been replaced by some other directsound; and in the third situation, no direct sound is being produced. Y

In the Iirst situation, the voiced sound whose spectrum is shown in FIG.2A will be taken as an example of an original direct sound that is stillbeing produced by the source at the time that the reverberant soundarrives at microphone 1. As previously noted, the components of thereverberant sound coincide in frequency with certain components of theemitted sound from loudspeaker 6, but the components of the reverberantsound differ in frequency by Af cycles per second from the correspondingcomponents of the direct sound arriving at microphone 1, owing to thechange in frequency introduced by frequency shifter 3 during the passageof the lirst part of the original sound through the system of thisinvention. As illustrated in FIG.l 2D, the amplitude spectrum of thecombined direct and reverberant sound arriving at microphone 1 thusincludes both the direct components shown in FIGS. 2A and thereverberant components, where the reverberant components are indicatedby smaller vertical lines that are displaced to the right on thefrequency scale by an amount Af from corresponding larger vertical linesrepresenting direct components. It is observed in a comparison of FIGS.2C and 2D that the first component of the emitted sound spectrum in FIG,2C is missing from the reverberant sound components in the combineddirect and reverberant sound spectrum in FIG. 2D, resulting fromcoincidence with a valley in the frequency response curve of the room.In addition, it is observed in FIG. 2D that the amplitudes of the lasttwo reverberant components fall below the gate threshold because oflosses in transmission over the feedback paths between loudspeaker 6 andmicrophone l.

' After microphone 1 converts the combined sound into an electricalwave, frequency shifter 3 changes the frequencies of both the directcomponents and the reverberant components of the Wave by an equal amountbut the relative displacement in frequency between direct'andreverberant components remains unchanged. From frequency shifter 3, therequency shifted wave is delivered to suppressor 4, where the wave isseparated into its individual components by filters da through 4in.Because of the difference in frequency between the reverberant anddirect components, it is highly probable that a reverberant componentand its corresponding original component will lie Within the pass bandsof different bandpass filters, if not on the first trip of thereverberant components around the feedback loop, then on a succeedingtrip. For example, if the contiguous pass bands of filters 41a throughdln are of a uniform thirty cycles per second Width, and the frequencyshift introduced by frequency shifter 3 is ten cycles per second, thenat most three trips around the feedback loop would suiiice to cause areverberant component and its corresponding direct component to liewithin the pass bands of different filters.

When a reverberant component does 'not lie in the same pass band as thecorresponding direct component, it is apparent that the spectrum of thereconstructed wave formed by adder 46 will contain the reverberantcomponent only when its amplitude exceeds the gate threshold. Because ofthe transmission losses previously mentioned, it is a relatively rareVoccurrence for the amplitude of a reverberant component to exceed asuitably high gate threshold. Equally important, even if a reverberantcomponent does have sufficient amplitude to appear in the spectrum ofthe reconstructed Wave, the change in frequency introduced by frequencyshifter 3 moves such a component to` a different point on the roomfrequency response curve, thereby making it very likely that thereverberant component will coincide with or fall near a valley in theresponse curve and be eliminated. Frequency shifter 3 thus operates toeliminateireverberant sound as a source of acoustic feedback instabilityinV two ways: by separating each reverberant component from itscorresponding Vdirect componentto facilitate suppression of individualreverberant components in suppressor`4and by moving each reverberantcomponent to a different point on the room frequency response curve tomake the reverberant components coincide with valleys in the roomresponse curve. It is clear that with each trip around the feedbackloop, the probability of eliminating reverberant components increases asthe change in frequency introduced by shifter 3 cuinulativelyincreases.V

In the event that a reverberant component does lie in the same pass bandas its corresponding direct component, the resulting control signalappearing at the output terminal o f the following low-pass lterrepresents the sum of the amplitudes of the vtwo components, andtherefore Y it is very probable that both the direct and the reverberantcomponents will appear in the spectrum of the reconstructed wave. Butone or two subsequent ltrips around the feedback loop will ordinarilysutiice to eliminate such reverberant components, either by coincidencewith a valley in the room frequency response curve, or by suppression insuppressor 4, or by a combination of coin-- cidence and suppression.

In the second and third situations, where reverberant sound arrives atmicrophone l either in time coincidence` with a direct sound differentfrom the sound from which the revcrberant sound is derived, or in theabsence of direct sound, respectively, the present invention operateseven more effectively to prevent acoustic feedback instability. For thecase in which the reverberant sound coincides in time with a differentdirect sound, it is highly improbable that a reverberant component willlie in` the pass band of a bandpass filter 41a through 4in which alsocontains a direct sound component. Thus in the: second situation, it ishighly probable that reverberantt components will be suppressed insuppressor 4 on a first trip around the feedback loop. Those reverberantcomponents which by chance lie in the same pass band as a directcomponent will eventually be eliminated in the course of one or twosubsequent trips around the feedback loop by the same mechanisms as inthe first' situation.

It is clear that in the third situation there are no direct componentsto aid reverberant components in passing through suppressor 4, andtherefore only in the comparatively rare instance when the amplitude ofa reverberant component exceeds the gate threshold will a reverberantcomponent appear in the spectrum of the reconstructed wave. In the eventthat a reverberant component does pass through suppressor 4, it issubject to the same processes of elimination on subsequent trips aroundthe feedback loop as reverberant components in the two precedingsituations.

lt is to be understood that the above-described arrangements are merelyillustrative of applications of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

l. In a sound amplification system comprising a microphone, a poweramplifier, and a loudspeaker,

means in circuit relation with said microphone, amplifier andloudspeaker for preventing instability due to acoustic feedback betweensaid loudspeaker and said microphone including a frequency shifter forchanging the frequency of each frequency component in the amplitudespectrum of an incoming wave by a uniform, predetermined amount, to forma frequency shifted wave having an amplitude spectrum with frequencycomponents that differ from the corresponding frequency components ofthe spectrum of said incoming wave by said predetermined amount offrequency change, and means connected to said frequency shifter forindividually suppressing in the spectrum of said frequency shifted waveeach frequency component whose amplitude is less than a preassignedthreshold.

2. A sound amplification system comprising a transducer located in asound field for converting audible sound into an electrical wave,

means for applying said electrical wave to a frequency shifter forchanging the frequency of each component of an incoming electrical waveby a predetermined, constant amount to form a frequency shifted wave,

means for delivering said frequency shifted wave to a suppressing means,said suppressing means including means for separating said frequencyshifted wave into its individual components,

means for obtaining from each of said components a control signalrepresentative of the amplitude of each component,

gating means responsive to said control signals and supplied with saidfrequency shifted wave for passing only those components of said wavewhose amplitudes exceed a preassigned threshold, and

means for reconstructing an electrical wave from the components passedby said gating means,

means connected to said suppressing means for delaying saidreconstructed wave for a selected time interval,

amplifying means connected to said delaying means for increasing thepower of said delayed reconstructed electrical wave to form an amplifiedelectrical wave, and

reproducing means located in the same sound field as said transducermeans for converting said amplified electrical wave into audible sound.

3. Apparatus for preventing instability due to acoustic feedback in asound amplification system including a microphone, a power amplifier,and a loudspeaker which comprises frequency shifting means for changingthe frequency of each frequency component in the amplitude spectrum ofan incoming electrical wave from said microphone by a uniform,preassigned amount to develop a frequency shifted wave having anamplitude spectrum in which each frequency component differs from thecorresponding frequency component in -the spectrum of said incomingelectrical wave by said preassigned amount of frequency change, and

means coacting with said frequency shifting means for individuallysuppressing in the spectrum of said frequency shifted wave eachfrequency component whose amplitude is less than a predeterminedthreshold and for individually passing unaltered to said power amplifiereach frequency component in said frequency shifted wave spectrum whoseamplitude is greater than said threshold.

. A sound amplification system comprising transducer located in a soundfield for converting audible sound into an electrical wave, meansconnected to said transducer for shifting the frequency of eachcomponent of an incoming electrical wave by a uniform, preassignedamount to develop a frequency shifted wave,

means for applying said frequency shifted wave to a suppressing meanswhich includes a plurality of subpaths, each of which is provided withan input terminal and an output terminal and each of which contains abandpass filter having a relatively narrow pass band for separating saidfrequency shifted wave into its individual frequency components, arectifier, and a low-pass filter connected in series,

a plurality of gating means in one-to-one correspondence with saidplurality of subpaths, wherein each gating means is provided with aninput terminal, an output terminal, and a control terminal having auniform, predetermined threshold,

means for connecting the output terminal of each subpath with thecontrol terminal of its corresponding gating means,

first delaying means provided with an input terminal and an outputterminal,

means for delivering said incoming frequency shifted wave to the inputterminal of each subpath and the input terminal of said rst delayingmeans,

means for connecting the output terminal of said delaying means to theinput terminal of each of said gating means, a plurality of bandpassfilters each of which is provided with a pass band corresponding to thepass band of the bandpass filter in a corresponding one of saidsubpaths, an input terminal connected to the output terminal of acorresponding gating means, and an output terminal,

adding means provided with a plurality of input terminals in-one-to-onecorrespondence with the output terminals of said plurality of bandpassfilters, and an output terminal, and

means for connecting the output terminals of said frequency shiftingmeans supplied with said electrical Wave for shifting each Vdirectfrequency compo- 1725566 S/z i Chetnut ""1 17g-ig nent and eachreverberant frequency component of 2401406 6/4 Bed ord et a 179- saidelectrical Wave by a uniform, predeterminedV 302'2504 2/62 Stroud et al'1791 amount of frequency to form a frequency shifted wave having anamplitude spectrum containing direct frequency components andreverberant fre- Y l@ s quency components that differ by saidpredetermined amount of frequency from the corresponding directfrequency components and reverberant frequency components in theamplitude spectrum of said elec- 5 trical wave, plurality of bandpassfilters to the corresponding means connected to said frequency shiftingmeans for input terminals of said adding means, suppressingsubstantially all of said reverberant fresecond delaying means providedwith an input terminal quency components in the amplitude spectrum ofand an output terminal, said frequency shifted Wave, said suppressingmeans means for connecting ,the output terminal of said addincluding ingmeans .to the input terminal of said second de-v 'Y means for separating.the spectrum of said frequency laying means, shifted wave into itsindividual frequency compopower amplifier means connected to the outputternents, and Y minal of saidvsecond delaying means, and meansVresponsive to said individual frequency cornreproducing means followingsaid power amplifier ponents for individually suppressing in thespectrummeans for converting an electrical Wave from said of said frequencyshifted Wave each frequency compower am'pliiermeans into audible sound.ponent having an amplitude that is smaller than a- 5. Apparatus forpreventing instability in a sound preassigned .threshold and forindividually transmitamplification system due to acoustic feedbackbetween ting each frequency component in said frequency a loudspeakerand a microphone of said system which shifted wave spectrum which has anamplitude that comprises exceeds said threshold to form the spectrum ofa a microphone located in a sound field of bothdirect reconstructedwave, f

sound and reverberant sound for converting said amplifier means suppliedwith said reconstr-ucted wave direct sound and said reverberant soundinto an for amplifying said reconstructed rwave, and electrical wavehaving an amplitude spectrum conloudspeaker means connected to saidamplifier means taining direct frequency components that correspond forreproducing audible sound from said amplified to the frequencycomponents of said direct sound reconstructed Wave. and reverberantfrequency components that corre-` f spond to the frequency components ofsaid rever- Ref'ellces Cited by the Examiner befat Sound, UNITED STATESPATENTS ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner.

1. IN A SOUND AMPLIFICATION SYSTEM COMPRISING A MICROPHONE, A POWERAMPLIFIER, AND A LOUDSPEAKER, MEANS IN CIRCUIT RELATION WITH SAIDMICROPHONE, AMPLIFIER AND LOUDSPEAKER FOR PREVENTING INSTABILITY DUE TOACOUSTIC FEEDBACK BETWEEN SAID LOUDSPEAKER AND SAID MICROPHONE INCLUDINGA FREQUENCY SHIFTER FOR CHANGING THE FREQUENCY OF EACH FREQUENCYCOMPONENT IN THE AMPLITUDE SPECTRUM OF AN INCOMING WAVE BY A UNIFORM,PREDETERMINED AMOUNT, TO FORM A FREQUENCY SHIFTED WAVE HAVING ANAMPLITUDE SPECTRUM WITH FREQUENCY COMPONENTS THAT DIFFER FROM THECORRESPONDING FREQUENCY COMPONENTS OF THE SPECTRUM OF SAID INCOMING WAVEBY SAID PREDETERMINED AMOUNT OF FREQUENCY CHANGE, AND